/Engine/Source/Runtime/SlateCore/Public/Widgets/SWindow.h
Returns whether or not this window is a descendant of the specfied parent window
true if the window is a child of ParentWindow, false otherwise.
bool IsDescendantOf ( const [TSharedPtr](API\Runtime\Core\Templates\TSharedPtr)< [SWindow](API\Runtime\SlateCore\Widgets\SWindow) > & ParentWindow ) const